The present invention relates to a method and system for controlling transverse profiles of a web material in a machine which manufactures the same, such as a board or paper machine, and/or a finishing machine which treats the web material. In the invention, the transverse caliper and grammage profiles are measured and actuators acting upon the profiles are regulated by means of the measurements of these profiles. Also, the moisture profile of the web can be measured and an actuator such as a steam box regulated to affect the moisture profile based on a measured tension profile of the web.
In the prior art, the various property profiles of a web manufactured or treated by means of machines for handling a web material, in particular by means of paper machines and by means of on-line finishing devices connected to the paper machines or by means of separate off-line finishing devices, web profiles such as the grammage, fiber-orientation, moisture, and thickness profiles, are controlled by means of various modes of regulation. The overall aim of the regulation is a final web product having a quality as high as possible and good runnability of the machines.
Earlier, the significance of the transverse tension profile of a paper web both for the final product and for the runnability of the machines has been realized, but, with regard to their fields of application, the prior art methods for controlling the tension profile of the web have been small-scale partial solutions, in which the papermaking process and its optimization both in view of the final product produced and in view of the runnability of the machines have not been taken into account as a whole.
In prior art paper machines and finishing machines, attempts are made to produce a web whose moisture, grammage, caliper, and fiber-orientation profiles in the cross direction are as uniform as possible. The prior art manufacturing processes accomplish this objective typically so that the tension of the web to be produced in the cross direction of the web varies to a substantial extent, e.g., whereby the tension in a middle area of the web is about 300 N/m to about 400 N/m and in lateral areas of the web the tension is about 100 N/m, which results in the drawbacks which will be described in more detail later.
The starting point and a critical basis of the invention has been the realization of the significance of a uniform tension profile of the paper web, which significance has been divided into five factors in the following discussion:
1. runnability in the paper machine;
2. quality of the machine reel;
3. runnability and quality of the roll from the slitter-winder;
4. runnability in finishing; and
5. quality of the final product.
The factors listed above will be dealt with in more detail in the order listed above.
1. Since every portion of the web must always be in a tensioned state in the machine direction in a paper machine, when the web running takes place with a uniform tension, the average tension can be minimized and, thus, the preferred draw differences and web tension can be determined specifically for each paper grade. For example, when a maximal stretch at break is the desired goal, the web should be run preferably with a low tension level.
2. In view of the quality of the machine reel, the uniformity of the caliper profile, i.e., the thickness profile, of the web or a deviation from this profile in a specified manner is the most important factor. A second important factor is usually a uniform moisture profile of the web. A third factor in the order of importance is the tension profile. When the caliper profile and the moisture profile of the web are uniform, the moisture profile is generally passed on to the tension profile of the roll produced in the slitter-winder.
3. Unevenness and above all obliqueness in the tension profile of the slitter roll cause various disturbances of deformation depending on the paper grade produced. For example, in sack paper having a low modulus of elasticity and high stretching ability, an oblique tension profile results in convex/concave-headedness of the rolls, which has been established experimentally. Slack web portions have a detrimental effect on the runnability of the slitter, because slackness cannot be permitted at the blade.
4. In newspaper printing shops, it has been noticed that an uneven tension profile causes, among other things, wrinkles and faults in the alignment of colors. If the properties of a roll change to a great extent on exchange of a roll, a break in connection with the roll exchange is more probable. From experience, it is known that in sack factories, such a fault in tension profile when occurring is seen as unevenness of a solidified stretch (a paper that has been cut into sheets does not form a plane face in the non-tensioned state), which is particularly harmful when making the sack bottom.
5. Since the tension of the web during drying affects its strength properties, such as tensile stiffness, tensile energy absorption, elongation, and equivalent, an uneven tension profile results in non-homogeneous strength properties in the profile of the web in the cross direction.
The problems and the requirements of further development that have occurred in the prior art described above and in corresponding prior art have contributed to providing motivation for the present invention.
The control of various transverse profiles of a web is made particularly problematic by the fact that all profiles act upon other profiles. In respect of the complex nature of this matter, reference is made to the assignee's Finnish Patent No. 81,848, corresponding to English-language European Patent Publication No. 0 408 894, which describes the interdependence and control of the grammage and fiber-orientation profiles of paper.